Separation of fatty acids



United States Patent SEPARATION OF FATTY ACIDS Judson G. Smull, Bethlehem, Pa., assignor to National Lead Company, New York, N. Y., a corporation of New Jersey No Drawing. Application May 10, 1956 Serial N0. 583,936 M 4 Claims. (Cl. 260-419) This invention relates to a process for the separation and purification of unsaturated fatty acids, particularly fatty acids in mixtures thereof, as in natural vegetable oils and the like. In particular, this invention relates to the separation of singly-unsaturated from plurally-unsaturated fatty acids. I

It is well known that singly-unsaturated fatty acids and poly-unsaturated fatty acids have substantially differing chemical properties, which adapt them particularly to a variety of uses for which one or the other is most appropriate. Thus, poly-unsaturated acids tend to be more reactive, particularly, when the double bonds are present in'a conjugated configuration, and all of the natural drying oils will be found to contain majorproportions of doubly-unsaturated fatty acid glycerides, usually either in conjugated configuration or in positions readily capable of rearranging to a conjugated configuration. On the other hand, singly-unsaturated fatty acids and their soaps, glycerides, etc., being less reactive, are frequently better adapted than their poly-unsaturated analogues for that very reason, ire. in systems where a lower degree of reactivity is desired. With few exceptions, however, these two classes of fatty acids or fatty acid derivatives are found in nature not separately, but rather. in varying degrees of admixture with each other. It has, therefore, long been customary to select as raw materials, naturallyoccurring substances richer in the class of fatty acid compounds desired, and relatively poor in the type not desired, using such criteria as iodine-number and the like to estimate the approximate degree of admixture and the properties, that may be expected from that particular raw material.

It has long been recognized that a source of relatively 1 pure fatty acids would be of major assistance in com: pounding and formulating, but such materials have not heretofore been readily available at any reasonable price.

Such methods as have been suggested for the production of pure fatty acids have generally involved longand complicated multiple-step separations or tedious and difiicultlycontrolled chemical operations such as selective hydrogenation or the like.

It is therefore an object of this invention to provide an improved, method for the purification and separation Another object is to provide a method prises dissolving said mixture in an alkaline medium.

comprising (1) an aliphatic alcohol selected from the group consisting of methanol and ethanol, in amount from 60 to 90% by volume of said medium, and (2) an alkali metal hydroxide in amount from 5 to by weight of said medium, the balance of said medium being ice substantially all water, said alkaline medium being em ployed in amount suificient to provide a quantity of said alkali metal hydroxide from to 60% of that stoichiometrically equivalent to the total fatty acid content of said mixture, chilling said solution to a temperature between 0 and 8 C., thereby precipitating an acid soap of said singly-unsaturated acid, separating said acid soap from the remainder of said solution, and converting said acid soap to a fatty acid.

, Although this method is a general one and may be used for the separation of singly-unsaturated from polyunsaturated fatty acids in general, this invention particularly contemplates the separation of substantially pure oleic acid (singly-unsaturated) from a mixture. of refined tall oil fatty acids, the non-oleic portion of which is principally linoleicuacid. The further description of this invention will therefore be largely in terms of the extrac tion of pure oleic acid from tall oil fatty acids. i

It has beenfound that when a mixture of singlyand poly-unsaturated fatty acids, such as refined tall oil fatty acids, is subjectedto the operations just described, the fatty acids, such as oleic acid, form acid soapsor semi-. soaps, which are believed to be weakly-bonded compounds consisting of one molecule of the fatty acid united by hydrogen bonding to one molecule of the alkali soap of the acid. Some variation in the relative proportions is apparently possible, as good results may be obtained the case of the singly-unsaturated acids, involve singlyunsaturated acids only.

Acid soaps of monocarboxylic acids are not unknown in the literature. It is known that the acids form molecular combinations with the sodium and potassium salts of the acids-thus the combination of formic acid, acetic acid and other monocarboxylic acids of higher molecular weight with their respective sodium or potassium salts has been observed. The preparation of potassium hydrogen dioleate, a well-defined crystalline substance, and

also that of potassium acidlauratqha ve been reported. The association of two mols of carboxylic acid, through hydrogen bonding to form a dimer, appears to explain the ability of these monocarboxylic acids to form acid salts. To my knowledge, however, there has heretofore been no recognition of the fact that the properties of the acid soaps of singly-unsaturated fatty acids exhibit a characteristic difference from those of the acid soaps of both saturated acids and poly-unsaturated acids, nor has there been any suggestion that a convenient and economical separation method might be based on the differences.

Specifically, it has been found that when the fatty acids in a mixture thereof are converted to their respective acid potassium or preferably sodium soaps in the manner here described, and the mixture containing the acid soaps is then chilled to a temperature between. t) and 8 C.,the solubilities of the acid soaps are such that the acid soaps of singly-unsaturated acids precipitate and may be sepa-, rated by filtration, decantation or the like, the acid soaps of the poly-unsaturated acids and of the saturated acids remaining in solution. Having effected the separation, it is a simple matter to convert theacid soaps back to the free acids.

Methanol and ethanol are operable as the alcohol portion of the alkaline medium. Of these, methanol is preferred, as the oleic acid semi-soaps are less soluble therein, and recovery of these soaps upon chilling is correspondingly higher. Isopropanol has been tried, but the yield of oleic acid soap obtained upon chilling from an alkaline isopropanol medium is too small to be of practical significance.

The amount of alcohol present in the alkaline medium should be between 60 and 90% by volume of the medium. When the amount of alcohol is lower than 60%, the tall oil acids will not readily go into solution. When the proportion of alcohol is too high, on the other hand, the solubility of the semi-soap is increased, and the recovery on chilling is diminished. v

The invention is specific to alkali metal hydroxides, the closely-related alkaline earth metal hydroxides being quite insoluble in the alcohol-water solvent mixture. The amount of alkali metal hydroxide should be between about 5 and about 15% by weight of the alkaline medium, i. e. of the total of water, alcohol and alkali. Where the concentration of alkali metal hydroxide is less than about 5%, unduly large amounts of the alkaline medium are required, in order to provide a sufficient amount of alkali metal to react with the oleic acid or the like. If

the hydroxide is present in too strong a concentration,

the acid soap will be insoluble at room temperature, and the product obtained under these conditions is less pure than one which is precipitated by cooling the solution, in addition to the likelihood of precipitating compounds of the saturated or poly-unsaturated acids. Of the common alkalis, that is to say sodium and potassium hydroxides, the former is greatly preferred, for, while potassium hydroxide is operable and capable of producing a useful separation, sodium hydroxide is both cheaper and more eifective, the acid sodium soaps of singly-unsaturated acids being much less soluble under the conditions employed than are the acid potassium soaps.

The total amount of alkali metal hydroxide relative to the amount of fatty acids present is critical; if less than about 25% of the stoichiometric quantity to react with said acid (to form the normal soaps) is present, the formation of the acid soap or semi-soap is incomplete, and the residual free fatty acid remains in solution, instead of being thrown down on cooling. If more than about 60% of stoichiometric is employed, on the other hand, the product will be a mixture of semi-soap and normal soap, and the latter will likewise fail to be thrown down on cooling.

Finally, the temperature employed in the chilling step is important. Temperatures above about 8 C. result in poor separation and poor recovery of the oleic acid. Preferably, the temperature at the cooling step should not exceed about 5 C. Too low a temperature, however, is also to be avoided, as it tends, to favor precipitation of other substances besides the desired acid soap, and thus adversely affects the separation.

The conversion of the acid soap to the free fatty acid is a conventional step, and may be accomplished in a variety of ways known to the art. One of the simplest and most convenient of such ways is simply to treat the acid soap with dilute (e. g. 5%) HCl.

In order to more'fully illustrate the nature of the present invention and the manner of practising the same, the following example is presented Example 270 grams of tall oil fatty acids were dissolved in 300 cc. of methanol. A separate solution was prepared containing 0.0656 grams of NaOH per cc., dissolved in a mixture of 20% water, 80% methanol.

By calculation from the analytically-determined acid number of the tall oil acids, it was determined that the amount of NaOH required to react stoichiometrically with the above quantity of tall oil acids, forming normal soaps, was 37.46 grams. Half of this quantity, or 18.73 grams NaOH, would be required to form the corresponding semi-soap. Accordingly, 285.5 cc. of the NaOH solution were added to the alcoholic solution of tall oil acids. The resulting mixture was allowed to stand at 3 C. for 16 hours. At the end of this period, the solution was examined, and a curdy precipitate was observed. The solution was then filtered on a cold Buchner funnel, and the filter cake was pressed to squeeze out excess liquid, and washed with 130 cc. of cold methanol-water mixture (:20% by volume). The acid soaps in the solid and in the liquid filtrate were converted to fatty acids by treating with an excess of 5% HCl and washing to remove NaCl. The product from the solid fatty acid soap was substantially pure oleic acid, which was recovered in good yield. The fatty acid derived from the filtrate was primarily linoleic acid, containing minor amounts of other materials carried through from the original tall oil fattyacids.

While this invention has been described with reference to certain preferred embodiments and by way of certain specific examples, these are illustrative only, and the invention is not to be limited, except as set forth in the following claims.

I claim:

1. A method for separating oleic acid from linoleic acid in a mixture thereof which comprises dissolving said mixture in an alkaline medium comprising (1) an aliphatic alcohol selected from the group consisting of methanol and ethanol, in amount from 60 to by volume of said medium, and (2) an alkali metal hydroxide in amount from 5 to 15% by weight of said medium, the balance of said medium being substantially all water, said alkaline medium being employed in amount sufiicient to provide a quantity of said alkali metal hydroxide from 25 to 60% of that stoichiometrically equivalent to the total fatty acid content of said mixture, chilling said solution to a temperature between 0 and 8 C., thereby precipitating an acid soap of said oleic acid, separating said acid soap from the remainder of said solution, and converting said acid soap to the fatty acid.

2. A method according to claim 1, in which said alcohol is methanol.

3. A method according to claim 1, in which said alkali 7 medium, and sodium hydroxide in amount from 5 to 15 by weight of said mixture, the balance of said medium being substantially all water, said alkaline medium being employed in amount sufiicient to provide a quantity of said alkali metal hydroxide between 25 and 60% of the quantity stoichiometrically equivalent to the total'fatty acid content of said mixture, chilling the resulting solution to a temperature between 0 and 8 C., thereby precipitating sodium acid oleate from said solution, separating said sodium acid oleate from the remaining solution, acidifying said sodium acid oleate with an acid having a water-soluble sodium salt, and washing with water to remove said water-soluble sodium salt.

References Cited in the file of this patent Brown et al.: J. A. C. 8., vol. 59, page 6 (1937).

Ralston: Fatty Acids and Their Derivatives, published by Wiley & Sons (N. Y.), 1947, (pages 287 and 288 relied on). I 

1. A METHOD FOR SEPARATING OLEIC ACID FROM LINOLEIC ACID IN A MIXTURE THEREOF WHICH COMPRISES DISSOLVING SAID MIXTURE IN AN ALKALINE MEDIUM COMPRISING (1) AN ALIPHATIC ALCOHOL SELECTED FROM THE GROUP CONSISTING OF METHANOL AND ETHANOL, IN AMOUNT FROM 60 TO 990% BY VOLUME OF SAID MEDIUM, AND (2) AN ALKALI METAL HYDROXIDE IN AMOUNT FROM 5 TO 15% BY WEIGHT OF SAID MEDIUM, THE BALANCE OF SAID MEDIUM BEING SUBSTANTIALLY ALL WATER, SAID ALKALINE MEDIUM BEING EMPLOYED IN AMOUNT SUFFICIENT TO PROVIDE A QUANTITY OF SAID ALKALI METAL HYDROXIDE FROM 25 TO 60% OF THAT STOICHIOMETRICALLY EQUIVALENT TO THEE TOTAL FATTY ACID CONTENT OF SAID MIXTURE, CHILLING SAID SOLUTION TO A TEMPERATURE BETWEEN 0 AND 8*C., THEREBY PRECIPATING AN ACID SOAP OF SAID OLEIC ACID, SEPARATING SAID ACID SOAP FROM THE REMAINDER OF SAID SOLUTION, AND CONVERTING SAID ACID SOAP TO THE FATTY ACID. 